pyserial and matplotlib imported at toplevel in several modules where not necessary

`matplotlib` is imported at a top level in `mkidgen3.server.waveform` (and `mkidgen3.plotting` and `mkidgen3.g3_paper` but that's fine if those don't work without the plotting feature)

`pyserial` is imported at a toplevel in the ifboard driver which creates issues for a server only install via the same mechanism as #14 